Gibberellins are a large family of tetracyclic triterpenoid plant growth hormones which promote various growth and developmental processes in higher plants, such as seed germination, stem elongation, flowering and fruiting (Stowe, B. B. et al., Annu. Rev. Plant Physiol. 8:181-216 (1957)). A number of GA responsive dwarf mutants have been isolated from various plant species, such as maize, pea, and Arabidopsis (Phinney, B.O. et al., "Chemical Genetics and the Gibberellin Pathway" in Zea mays L. in Plant Growth Substance, ed., P. F. Waering, N.Y.: Academic (1982) pp. 101-110; Ingram, T. J. et al., Planta 160:455-463 (1984); Koornneef, M., Arabidopsis Inf. Serv. 15:17-20. (1978)). The dwarf mutants of maize (dwarf-1, dwarf-2, dwarf-3, dwarf-5) have been used to characterize the maize GA biosynthesis pathway by determining specific steps leading to biologically important metabolites (Phinney, B. O. et al., "Chemical Genetics and the Gibberellin Pathway" in Zea mays L. in Plant Growth Substance, ed., P. F. Waering, N.Y.: Academic (1982) pp. 101-110; Fujioka, S. et al., Plant Physiol. 88:1367-1372 (1988)). Similar studies have been done with the dwarf mutants from pea (Pisum sativum L.) (Ingram, T. J. et al., Planta 160:455-463 (1984)). GA deficient mutants have also been isolated from Arabidopsis (gal, ga2, ga3, ga4, ga5) (Koorneef, M., et al., Theor. Appl. Genet. 58:257-263 (1980)). The Arabidopsis ga4 mutant, induced by ethyl methanesulfonate (EMS) mutagenesis, is a germinating, GA responsive, semidwarf whose phenotype can be restored to wild type by repeated application of exogenous GA (Koornneef, M. et al., Theor. Appl. Genet. 58:257-263 (1980)).
In Arabidopsis, the ga4 mutant allele blocks the conversion of 3-.beta.-hydroxy GAs, reducing the endogenous levels of GA.sub.1, GA.sub.8 and GA.sub.4 and increasing the endogenous levels of GA.sub.19, GA.sub.20 and GA.sub.9 (Talon, M. et al., Proc. Natl. Acad. Sci. USA 87:7983-7987 (1990)). The reduced levels of the 3-.beta.-hydroxy GAs is the cause of the semidwarf phenotype of the ga4 mutant. It has been suggested that the pea le mutant also encodes an altered form of 3-.beta.-hydroxylase (Ross, J. J. et al., Physiol. Plant. 76:173-176 (1989)).